Pavement Analysis

 

PTS Limited are a specialist consultancy in the measurement and utilisation of road network survey data.

Analytical Pavement Analysis:

Using multi-layer elastic theory to determine the interactions between the different pavement layers, the stresses/strains generated in the pavement are calculated for the required loading (design vehicle). Based on typical properties of the pavement materials, relationships can be obtained between the stress/strain criteria and the number of load applications before the pavement reaches its design life.

Wherever possible materials testing is performed on samples cored from the pavement to assist in characterisation of material behaviour. The results from the Indirect Tensile Stiffness Modulus test (ITSM) in the Nottingham Asphalt Tester (NAT) provide data on stiffness at different temperatures and/or strains, which is used to assist interpretation of the FWD data. The Repeated Load Axial Test (RLAT) is used to define visco-elastic material properties. The RLAT data is then used, in a multi-layered viscous model of the pavement, to predict the progression of non-structural rutting within the bituminous materials. A third test in the NAT, the Indirect Tensile Fatigue Test (ITFT), helps to define the resistance of the material to fatigue cracking, and is incorporated in a method for assessing reflection cracking of bituminous materials in composite pavements (ie. Pavements with a bituminous overlay on a concrete or cement bound base).

For concrete slabs the life of the pavement is assessed by calculating the stresses which are induced under the passage of the design loading. The most significant stresses are the horizontal tensile stress at the bottom of the concrete in a slab centre location, and the tensile stress at the top of the concrete at an edge or corner location. The former is calculated using the pavement model determined from the back-analysis, and then factors are applied (based on developments of the Westergaard equations) to determine the edge or corner stresses. The stresses caused by restraint of temperature induced movements can also be considered. The life prior to onset of fatigue cracking can then be determined from the ratio of induced stress to concrete strength. For a lean concrete base, the design criterion relating to fatigue is maximum tensile stress at the bottom of the layer.

If the residual life of the existing pavement is inadequate, the effect of different overlay thickness on the overall life of the pavement can be obtained for a selection of overlay materials, and suitable remedial measures designed. Alternatively, full or partial reconstruction options can be assessed, using stiffnesses for the lower pavement layers and subgrade which are based on the back-analysed value